This invention concerns a method for processing negative photoresist emulsion, and more particularly, a method of keeping a film of negative photoresist emulsion in an atmosphere of gas not containing oxygen after exposure of said photoresist to electron beams and the like, and an apparatus for carrying out such a method.
As is generally known, the ability to form fine patterns of submicron size on a semiconductor substrate has been increasingly required to keep pace with the trend of high integration and maximum function density of semiconductor devices such as integrated circuits (IC). In preparing such fine patterns by the so-called mask process method employing the photolithography technique, the known art process of exposing the substrate to ultraviolet light has been found ineffective to form sufficiently fine patterns. Thus, various other methods of exposure to electron beams, X-ray beams, ion beams or deep ultraviolet light are being developed and actually practiced. However, many of these new exposure systems require a considerable amount of time. For example, in an electron beam direct pattern writing method in the field of electron beam exposure wherein electron beams directly hit the photoresist film formed on a semiconductor substrate or wafer without using any mask, the exposure step takes from 50 or 60 minutes to several hours, although highly precise and fine patterns can be formed.
In the method just described, exposure of negative photoresist to an electron beam is carried out in vacuum. When a wafer coated with such photoresist is moved from vacuum to the atmosphere immediately after termination of exposure to electron beams, it has been found that the size of the pattern exposed to electron beams at an early stage of the exposure operation, and the size of the pattern exposed to electron beams immediately before termination of the exposure operation, differ when developed.
The difference in pattern size corresponding to the length of the curing time that elapses before developing is due to the difference in the bridging effect. There is a relationship, as shown in the logarithmic scale of FIG. 1, between the width W or thickness T of the written pattern, which is shown on the vertical axis, and the length of curing time t after exposure, which is shown in the horizontal axis. If a wafer is kept in vacuum in an electron beam exposure apparatus from the time At when the pattern writing started, to the time Bt when the pattern writing is finished, the width W or the thickness T of each line which was written first and last, respectively, takes the value T.sub.A and T.sub.B shown on the vertical axis when the wafer is taken out of vacuum relatively soon after the exposure. There is thus the difference (T.sub.A -T.sub.B) which is a substantial value in the widths or thickness of the lines written soon after the beginning and near the end of exposure. The phenomenon just described is called herein, the curing effect.
Despite the foregoing, if the wafer is kept for a considerable length of time in vacuum of the exposure apparatus after exposure of the wafer to electron beams, one hour for example, the time consumed in pattern writing becomes small relative to the time spent in the apparatus. In otherwords, since the time axis changes logarithmically, the change (T'.sub.A -T'.sub.B) during the time (At'-Bt') is small as shown in FIG. 1. This means that, when the wafer is kept in vacuum of the apparatus for a considerable length of time, there is practically very little change in the widths and thicknesses of the lines made in the first and last stages of the exposure.
In view of the facts described so far, measures must be taken to leave the wafer in vacuum for a certain length of time after the exposure is over. The above-described measure is necessary because even a slight difference in the pattern size is a significant defect in the formation of submicron patterns. However, such an operation delay drastically lowers the rate of operation of the electron beam exposure apparatus.
In order to eliminate the above-described drawback experienced with the conventional apparatus, an attempt was made to effectively utilize the exposure time by an apparatus wherein a part of the conventional apparatus is kept in vacuum and the substrates or wafers are successively transferred to that vacuum part. However, even with such a structure, handling of wafers in vacuum is extremely difficult and involves deterioration of the degree of vacuum. Because of this, there is another drawback in that the apparatus must be controlled very strictly.
The facts explained so far are scatteringly disclosed in various Japanese patent publications. Japanese patent publication (Unexamined) No. 124397 of 1978 published on Oct. 30, 1978 discloses an invention entitled: Transport Device. The apparatus according to the disclosed invention involves a technique for transferring articles irradiated with electron beams, ion beams, etc., in vacuum. In a paragraph on pages 1 and 2 of the specification in this publication, there appears a description that developments are being made to use electron beams and ion beams widely for treating and processing metals and other materials, for example, dissolution, blowout, welding, heat treatment, sputtering, etching, ion implantation and the like.
Japanese patent publication (Unexamined) No. 95189 of 1979 published on July 27, 1979 concerns an invention entitled: Electron Beam Exposure Device. On page 5 of the specification there is a description that a subchamber is kept air tight for pumping out air. Articles to be treated are transported from the sub-chamber which is thus kept in vacuum to an exposure chamber which also is kept in vacuum. On page 7 of the specification, there is a further explanation that the device is so arranged that once air is exhausted, the exposure process may continuously be made in vacuum.
Japanese patent publication (Unexamined) No. 68622 of 1980 published on May 23, 1980 discloses an invention entitled: Device for Exchanging Samples in an Apparatus for Electron Beam Pattern Writing. On page 4 of the specification carried therein is a description of a curing chamber which is kept in vacuum on the order of less than 10.sup.-4 Torr. The curing chamber is independent from a chamber where samples are loaded and unloaded, and its object is to carry out the curing of samples in vacuum according to the claim of the specification. On page 6 of the specification, there appears a description that when the pattern writing is over, samples are cured in vacuum without ejection of the mask to the outside atmosphere. Further, on page 7 of the specification, it is described that, for the purpose of improving exposure and stabilizing the width of the line of the pattern, curing in vacuum is necessary.
Japanese patent publication (Unexamined) No. 146932 of 1980 published on Nov. 15, 1980 is for an invention entitled: Device for Exchange of Samples in an Apparatus for Electron Beam Pattern Writing. On page 3 of the specification there is a description that in the photoresist that has been exposed to electron beams, polymerization progresses on the vacuum (in order of higher than 1.times.10.sup.-4 Torr), and the width of the line of the pattern increases. After a lapse of a certain length of time, this trend subsides, and when the photoresist is placed in vacuum of less degree or in the atmosphere, the polymerization ceases. According to a description on page 5 of the specification, curing occurs when pattern writing is applied to the sample. After completion of the curing, the sample is taken out of the curing chamber, and while maintaining the chamber in vacuum, the sample is transferred to the next stage of the process. On page 16 of the specification, it is described that the device of the invention allows maintaining high degree of vacuum required for the curing.
None of these publications, however, discloses or suggests the method of, and apparatus for, keeping the sample that has been exposed to electron beams in an atmosphere of non-oxidizing gas.